1. Field of the Invention
The present invention generally relates to methods for manufacturing a stacked gate structure in a semiconductor device. More particular, the present invention relates to a method for manufacturing a stacked gate structure in a field effect transistor.
2. Description of Related Arts
Chip manufacturers have always tried to achieve higher device operating speed. Reduction of sheet resistance and contact resistance of a gate electrode is an effective way to accomplish the aforementioned goal. Therefore, a poly-Si/WN/W gate is now regarded as a potential structure in DRAM technology beyond 0.18 μm generation. The WN layer is used as a barrier layer which prevents inter-diffusion between silicon atoms in the poly-silicon layer and tungsten atoms in the WN/W layers. The sheet resistance of such gate structure is lower than 10 Ω/□, which is better than that of the conventional poly-Si/WSi structure.
FIGS. 1A and 1B are cross sectional views setting forth a conventional method for manufacturing a poly-Si/WN/W gate structure. To begin, a gate dielectric layer 102, a poly-silicon layer 104, a barrier layer 106, a tungsten (W) layer 108, and a silicon nitride layer 110 are sequentially formed on a semiconductor substrate 100, as shown in FIG. 1A. Thereafter, a lithography process and an etching process are performed and then the silicon nitride layer 110 is patterned to form a predetermined configuration, thereby obtaining a hard mask pattern 110A. Subsequently, the tungsten layer 108, the barrier layer 106, the poly-silicon layer 104 and the gate dielectric layer 102 are patterned to form the predetermined configuration, thereby obtaining a gate structure provided with a patterned gate dielectric layer 102A, a patterned poly-silicon layer 104A, a patterned barrier layer 106A and a patterned tungsten layer 108A, as shown in FIG. 1B.
Conventionally, the method used to form a barrier layer 106 is to form a WNX layer on the poly-silicon layer. Then, a rapid thermal annealing (RTA) process is performed in an N2 ambient so that nitrogen atoms diffuse out from the WNX layer. As a result, the WNX layer is changed into the tungsten layer and a WN/SiN composite barrier layer is formed between the poly-silicon layer and the tungsten layer. However, SiN is an insulating material such that the resistance of the WN/SiN layer is raised.